A magnetic recording medium typically consists of a cured coating derived from a dispersion of magnetic particles in a binder coated on a non-magnetic support. In audio and video tapes, the tapes often contain a back coating for anti-static protection. The back coat may have the same composition as the front coat, or merely be a coating containing conductive carbon black. In the case of flexible magnetic recording disks, both sides of the non-magnetic support are usually coated with the dispersion of magnetic particles in a binder.
The binder typically consists of curable polymeric resins such as polyester urethanes, nitrocellulose, poly(vinyl chloride/vinyl acetate/vinyl alcohol), and poly(vinylidene chloride). In addition to binder and magnetic particles, the recording medium usually contains dispersant to help uniformly disperse the magnetic particles, and sometimes lubricant and a head cleaning agent.
A large percentage of currently produced magnetic media use polyester urethanes as part of the binder system. Polyester urethane elastomers are susceptible to environmental degradation by hydrolysis, a reaction which occurs between the recording medium and atmospheric water vapor. Hydrolysis of a polyester involves the action by water on the covalent bond of the ester functional group that links the backbone of the polymer.
Binder hydrolysis results in operational problems including the magnetic layer becoming sticky and gummy with resultant shedding of the sticky fragments. Also, frictional concerns result from the hydrolysis of the ester binder component, including increased stiction and decreased ability of the medium to slide smoothly over heads, guide pins, and other surfaces. A probable cause for the increased friction is the presence of carboxylic acid and alcohol moieties among the degradation products of hydrolysis. This increases the attractive interaction between the magnetic coating layer and the surfaces it contacts.
In addition to problems associated with ester hydrolysis, problems arise as the level of magnetic particle loading is increased. Modern development efforts aim at achieving very high data packing densities on a recording surface. Thus, greater density of magnetic particles is required. Generally, magnetic particle loading is considered high when the percent of magnetic oxides by weight of the solids is above about 72%.
The first problem encountered with increased densities of magnetic particles is the resultant decrease in space available for other necessary components. In other words, as more magnetic particles are used there is less and less space for binder and dispersant. If insufficient binder is used, problems such as loss of reproduction output and decreased durability result as magnetic particles are dislodged from the binder and deposited on the heads, guide pins, and other surfaces. The problems which occur when insufficient dispersant is used are discussed below.
Further, as the level of magnetic particles increases it becomes more difficult to homogeneously disperse the particles into the magnetic coating layer. In order for the magnetic recording medium to have optimal magnetic characteristics and electromagnetic transducing characteristics, the magnetic particles must be homogeneously dispersed within the magnetic coating layer. Thus, increased levels of magnetic particle loading dictate that increased levels of dispersant are used. Increased dispersant levels further compound the space constraints associated with increased particle loading.
Commonly used dispersants include higher aliphatic amines, higher fatty acids, esters of higher fatty acids, sorbitol, sodium alkylbenzenesulfonate and the like. Increased levels of dispersants used to disperse the higher levels of magnetic particles often result in an increase in the frictional force between the magnetic coating and the various surfaces the media contacts and results in the reduction in the strength of the magnetic coating. These effects result in problems with, for example, audio or video tapes, which include wow, flutter, jitter and skew, along with problems stemming from the weakened coating such as decreased output, head clogging, and frequent dropouts.
Magnetic recording media are generally prepared by coating a dispersion of the magnetic particles, binder, dispersant and other additives onto the non-magnetic support. In addition to the dispersibility difficulties which result from higher levels of particle loading, increases in the viscosity of the dispersion can make obtaining the required flat, smooth, magnetic surface more difficult. In the coating transfer step, during the manufacture of the magnetic recording medium, increased viscosity makes it increasingly defect to transfer the coating smoothly. Higher viscosities can also cause local retention of the coating in the transport path. In addition, dropouts may be formed and good surface properties may be difficult to obtain.
The viscosity of the dispersion is increased as the molecular weight of the binder polymers used is increased. Typically the binder systems include polymers having molecular weights in the range of 20,000 to 200,000. As set forth above, increased viscosity can have undesirable results. However, it has previously been difficult to obtain magnetic recording media with the desired performance properties from binder materials having low molecular weights. Previously, when low molecular weight polymers were used, it was difficult to get sufficient cross-linking; resulting in decreased performance characteristics, notably runability, and increased stiction.